Single ply and liquid
EPDM membrane sheet size with upstands and edge turn-downs
- where
- sheet length (m)
- roof length (m)
- allowances at each end of the length (mm)
- sheet width (m)
- roof width (m)
- allowances at each side (mm)
- sheet area (m²)
- flat roof area (m²)
Explain this equation
Works out the size of a one-piece EPDM rubber sheet so it reaches up the upstands and over the edges. Use it before ordering a cut-to-size membrane.
The flat roof is L by W. At each edge you need extra membrane: up an upstand and tucked into the chase, or down over a drip or trim. Those allowances are entered in millimetres, so dividing by 1000 adds them to the metre dimensions. The sheet area A_s is compared with the flat area A_r to show how much extra you are buying.
It is a rectangle grown by its edge allowances on each side.
Sense check: a 4 × 3 m roof with a 200 mm upstand at one end and 75 mm edges elsewhere needs about a 4.28 × 3.15 m sheet, around 12% more than the flat area.
Watch out: upstands generally need at least 150 mm above the finished roof plus the tuck into the wall. Order to the next stock size up.
Works out the one-piece EPDM sheet size to order from the roof size and the allowance needed at each edge for upstands, trims and turn-downs.
Enter your values to see the result.
Good to know
At an abutment allow the upstand height plus a small tuck or trim allowance: BS 6229 asks for the waterproofing to reach 150 mm above the finished roof, so 150 + 50 = 200 mm is a sensible minimum. At drip and gutter trims, manufacturers ask for the membrane to overhang the edge (ClassicBond: at least 75 mm; RubberBond: at least 50 mm all round). Order the next size up from the supplier’s stock widths and lengths.
Common mistake
Measuring the flat area only and forgetting the upstands. A 4 × 3 m roof with a wall on one side needs a sheet at least 4.275 × 3.15 m.
Limits
Single rectangular sheet. Larger roofs that need joined sheets need extra for laps (RubberBond asks for at least 100 mm). Kerbs, rooflights and penetrations need their own allowance.
Sources
- BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings – Code of practice (BSI)
- Bauder: BS 6229:2018 overview of changes (design 1:40 to achieve 1:80 finished; 150 mm upstands)
- ClassicBond EPDM product selector and installation guide (deck adhesive 2.8–4 m²/l; bonding adhesive 1.5–2 m²/l; 75 mm overhang)
- Flex-R RubberBond EPDM installation manual (50 mm minimum excess at perimeters; 100 mm sheet overlaps)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the roof covering manufacturer’s fixing specification, BS 5534, BS 8000-6, BS 6229, the Lead Sheet Training Academy guidance and Building Regulations. Wind uplift and structural design need a competent designer.